TOFO, I think you are confusing OASC and IOT.
I read MGD's "If you have the motivation and qualities they are looking for" as focusing on motivation and qualities at OASC. The corollary is that an excellent SNCO may be just that and lack the motivation and qualities that are perceived as essential for a commission.
A signicantly larger proportion of 'rankers' are commissioned compared with DE although many more aspiring DE will present.
The more junior airman and JNCO may also be prefered over the older SNCO as the latter may be more set in their ways than the younger aspirant.